ABSTRACT: A flow-through experiment was conducted on intact cores of sediments from Saginaw Bay, Lake Huron, to examine how trophic interactions between filter-feeding bivalve mussels and microbial populations could affect nitrogen dynamics at the
sediment-water interface. The zebra mussels used in this experiment removed a large proportion of protozoa and phytoplankton from the overlying water, particularly heterotrophic nanoplankton (up to 82%), while bacterial populations showed less change. A
3-fold decrease in the protozoan to bacterial carbon ratio corresponded to a 2.5-fold increase in relative ammonium removal rates as estimated from the dark loss of 15N-ammonium. Excretion by the bivalves also increased net ammonium flux to the
water, thus elevating the total calculated areal ammonium removal rates to about 6-fold over rates observed in the control treatment. These data suggest that filter-feeding bivalves may significantly affect nitrogen transformation rates near the
sediment-water interface by excreting ammonium and altering the microbial food web structure at the sediment-water interface.